(1) Field of the Invention
The present invention relates to a method for treating waste water containing oil composed of esters by adsorbing the oil from the waste water which to be released to an environment, and separating the oil from adsorbent as water soluble alcohols and acids by hydrolysis.
(2) Description of the Prior Art
KAGAKU KOUGAKU BENRAN (Chemical Engineering Handbook) Rev. 4th Edition, Maruzen Co. (Tokyo) (1978), Chapter 11, teaches the following prior art; Phosphoric acid-alkylesters, phosphonic acid-alkylesters, and phophinic acid-alkylesters, and the like, having a P--O bond in their molecule have a cation exchange function, and they are used for metal extraction from their aqueous solution. Tributyl phosphate, one of phosphoric acid-alkylesters, is widely used as a practical extraction solvent for extracting heavy metal ions from their aqueous solution in uranium refining processes, reprocessing processes of spent fuel, and the like.
Tributyl phosphate has solubility of about 400 mg/l in water at room temperature, and is distilled out accompanied with water vapor from an evaporator to process the liquid effluent. Accordingly, a possibility that waste water from related plants contain a small amount of tributyl phosphate can be assumed. Tributyl phosphate in the waste water is defined as "Mineral oil which is extractable by n-hexane" by Japanese regulation law, and the maximum concentration of such mineral oil in waste water which is allowable to release in environment is required to be 5 mg/l.
Y. Nojima et al (Operational Experience in the Low Level Liquid Waste Treatment at Tokai Reprocessing Plant, Proceedings of Fuel Reprocessing and Waste Management, August 26-29 (1984), Vol. 1 pp. 505-515) teaches the following prior art; In a Japanese reprocessing plant for spent fuel, waste water is treated with active carbon for eliminating tributyl phosphate by contacting and adsorbing in order to comply with the regulation, and then is released to the environment. The spent active carbon which has adsorbed the tributyl phosphate is stored in a storage at present, and will be treated and disposed later in other processes.
Methods for eliminating oils contained in water by adsorption into active carbon have been used widely, and the method itself is sufficiently established technology. The oil adsorbed spent active carbon is heated at about 1000.degree. C. in an oven, and can be finally reactivated by a gasification reaction. Accordingly, only deteriorated active carbon of which adsorbing capacity is decreased by repeating of the reactivation is treated for disposal. However, in case of tributyl phosphate as for the oil, the above described conventional reactivation of the active carbon can not be applied because heating of tributyl phosphate generates phosphoric acid, and consequently, spent active carbon is stored without reactivation and is accumulated. Generally speaking, waste active carbon is treated with an incinerating process first, and only ashes are disposed. However, the waste active carbon which adsorbed tributyl phosphate contains a large amount of phosphoric acid, and accordingly, it becomes necessary to be added with lime for protecting the oven wall in the incinerating process, therefore, a large amount of incineration residue containing calcium phosphate is generated.
As one of methods aiming at decomposing tributyl phosphate which was used as an extracting solvent, Schulz et al disclosed a method wherein tributyl phosphate was decomposed in butyl alcohol and water-soluble phosphoric acid compounds by heating the tributyl phosphate with 50% NaOH aqueous solution at the boiling point of 130.degree. C., and the butyl alcohol was separated by distilling. (W. W. Schulz, and J. D. Navratil, "Science and Technology of Tributyl Phosphate", Vol. I, Chapter 5, CRC Press Inc. (1984)).
Furthermore, as a method for decomposing tributyl phosphate solution chemically, JP-A 60-105997 (1985) discloses a method wherein radioactive spent organic phosphoric esters are oxidized to decompose by being heated with hydrogen peroxide in phosphoric acid aqueous solution of copper phosphate.
The above described treating methods required usage of a large amount of chemical agents in a condensed condition at elevated temperature, and accordingly, the methods could not be used for treatment of a large amount of waste water containing a very small amount of tributyl phosphate.
Tributyl phosphate is chemically stable material, but on account of its property as an ester of phosphoric acid, it has a tendency to hydrolyze under a water existing condition into butyl alcohol and dibutyl phosphate, monobutyl phosphate, and phosphoric acid. These products are all water-soluble and are not mineral oils extractable by n-hexane.
Hydrolysis of tributyl phosphate proceeds in the following three steps such as first dibutyl phosphate is generated with butyl alcohol, subsequently monobutyl phosphate, and finally phosphoric acid is generated. EQU (C.sub.4 H.sub.9 O).sub.3 PO+H.sub.2 O.fwdarw.(C.sub.4 H.sub.9 O).sub.2 PO(OH)+C.sub.4 H.sub.9 OH tributyl phosphate dibutyl phosphate butyl alcohol (C.sub.4 H.sub.9 O).sub.2 PO(OH)+H.sub.2 O.fwdarw.C.sub.4 H.sub.9 OPO(OH).sub.2 +C.sub.4 H.sub.9 OH dibutyl phosphate monobutyl phosphate butyl alcohol C.sub.4 H.sub.9 OPO(OH).sub.2 +H.sub.2 O.fwdarw.H.sub.3 PO.sub.4 +C.sub.4 H.sub.9 OH monobutyl phosphate phosphoric acid butyl alcohol
It is well known that hydrolysis of tributyl phosphate is accelerated by addition of acid or alkali. Under an acid existing condition, the above described hydrolytic reactions proceed parallel as primary reactions relating to concentration of each compounds, and reaction rate constants are in a ratio of about 4:2:1. The hydrolysis reaction rate constant of tributyl phosphate dissolved in 1N nitric acid aqueous solution is 4.times.10.sup.-6 /hour at 30.degree. C., and 1.times.10.sup.-4 /hour at 60.degree. C. Effect of nitric acid concentration on the hydrolysis reaction rate constant is scarce in a range between 1N and 8N. The hydrolysis reaction rate constant of tributyl phosphate contacting with 1N nitric acid (concentration of 0.86 gram molecule/l) is 3.3.times.10.sup.-6 /hour at 30.degree. C., and 1.1.times.10.sup.-4 /hour at 60.degree. C., and it does not differ so much from that of aqueous solution.
It is well known that hydrolysis of tributyl phosphate contacting with alkali aqueous solution proceeds as a primary reaction relating to concentration of tributyl phosphate and concentration of alkali in the aqueous solution, and the hydrolysis reaction hardly exceed the step of generating dibutyl phosphate. The hydrolysis reaction rate constant of tributyl phosphate dissolved in 1N sodium hydroxide aqueous solution is 5.5.times.10.sup.-3 /hour at 30.degree. C., and 6.times.10.sup.-2 /hour at 60.degree. C. The hydrolysis reaction rate of tributyl phosphate in alkali aqueous solution is larger than that in acid aqueous solution by about 1000 times. However, it is unrealistic to maintain a remarkably large amount of waste water to be treated in alkaline condition in consideration of a large amount of alkaline agents and secondary waste to be generated.
It is well known that hydrolysis of tributyl phosphate is accelerated by radiation exposure, and JP-A-3-178392 (1991) discloses a method wherein tributyl phosphate contained in waste water is decomposed to dibutyl phosphate by radiation exposure of the waste water. However, in order to give sufficient energy to a large amount of waste water, a large amount of radiation source is required and, consequently, it causes a problem that a large scale apparatus thereof is required.